Tropical Cambodia is located in Southeast Asia, between Vietnam, Laos, Thailand and the Gulf of Thailand. It's a largely jungle-clad low country, with two distinct geographical features: the Mekong river and Tonle Sap lake. The only truly mountainous regions are in Ratanakiri in the northeast, and the Cardamom range in the southwest, which is home to Cambodia's highest peak: Phnom Aural (1813 m).
Ratanakiri, in the northeast of Cambodia, is good for day-hikes and 2- to 4-day hikes to hill tribe villages, on foot and/or by elephant. In the same area you will find the wonderfully untouched Virachey National Park, where you have a choice of interesting multi-day treks through deep jungles to remote upland areas, and even along a part of the historic Hochiminh Trail, all organized through the park office in Banlung town.
The southwest of Cambodja is more easily accessible and has several national parks with hiking trails, although mostly day-hikes, like Bokor National Park in the Elephant (Damrei) Mountains, Kep National Park, Kirirom National Park near Phnom Penh, and Ream National Park near the coast.
Best time for hiking in Cambodia is during the dry season, from November to April.